Electroacoustic transducers belonging to the type of piezoelectric transducers have been well-known in the art for us as telephone receivers. In a piezoelectric transducer, which contains a piezoelectric layer attached to a transducer plate, a deflection of the transducer plate is produced when an electric voltage is applied across the piezoelectric layer attached to the transducer plate. This causes an acoustic pressure to be generated. The acoustic pressure so generated is generally proportional to the electric voltage applied thereto. Inversely, when the piezoelectric layer is subject to an acoustic pressure, an electric voltage will be created at the electrodes connected respectively to the opposite sides of the piezoelectric layer upon which the acoustic pressure impinges. The electric voltage thereby created is generally proportional to the acoustic pressure. Because of their light weight, volummetric compactness, and low electric power consumption, the piezoelectric receivers are gradually taking over the place of the traditional electromagnetic receivers.
An important consideration in the design and construction of piezoelectric receivers is to provide a level frequency response. Conventionally, this is done by providing a ring-shaped metallic bearing on the top or from the bottom of the transducer plate, or using an adhesive or a metallic housing having a supporting seat to support the transducer plate. Because of this requirement, conventional piezoelectric receivers require many additional accessory components, such as the ring-shaped metallic bearing, elastic rubber, metallic outer housing, etc. This not only complicates the assembly process, it also contributes to an increased manufacturing cost.
The basic construction of a piezoelectric transducer, or a piezoelectric receiver, can be found in U.S. Pat. No. 3,708,702, in which it is disclosed a transducer plate containing a piezoelectric layer which is supported at its periphery by a ring-shaped elastic body so that the fundamental frequency of oscillation of the transducer plate will have a node line in the region where the transducer is supported. The elastic supporting body, preferably made of silicon rubber, has two opposed pairs of inwardly projecting circular or ring-shaped projections, which engage with and support the transducer plate. The elastic supporting body is constructed so that it extends upwardly to substantial engagement with the outer cover of the transducer. The piezoelectric receiver disclosed in the '702 patent contains a large number of component parts, therefore, it incurs relatively high production costs. Furthermore, the piezoelectric receiver disclosed in the '702 does not contain provisions for connecting to a hearing aid.
U.S. Pat. No. 4,376,232 discloses an improved piezoelectric receiver over those disclosed in the '702 patent. In addition to the basic constructions, which include the ring-shaped elastic body and the inwardly projecting circular or ring-shaped projections, the '232 patent also discloses a plurality of Helmholtz resonators and an attenuation disc provided in the front chamber of the receiver for the attenuation of resonance increases. The piezoelectric receiver disclosed in the '232 patent involves more components than the '702 patent; and it does not contain provisions for connecting to a hearing aid.
U.S. Pat. No. 4,379,212 further improved the piezoelectric receiver disclosed in the '232 patent by replacing the customary silk disk, which is used as an attenuating material in the Helmholtz resonator, with a plurality of narrow slots arranged in the carrier plate. The piezoelectric receiver disclosed in the '212 patent also involves a substantial number of components thus would high production cost. Furthermore, it also does not contain provisions for connecting to a hearing aid so that the receiver can be used by the hearing impaired.
U.S. Pat. No. 4,779,246 discloses a piezoelectric receiver which basically follows the same design as those disclosed in the above mentioned U.S. patents. However, the piezoelectric receiver disclosed in the '246 patent utilizes an improved means for allowing the electrically conducting wires, which are connected to the electrodes of the transducer plate, to be connected to an external source. The piezoelectric receiver disclosed in the '246 allows the assembly procedure to be simplified; however, most if not all of the problems mentioned above remain unsolved.
U.S. Pat. No. 4,949,387 discloses an electroacoustic transducer which can be used either as a microphone, a receiver, or a voice frequency ringing transducer. This electroacoustic transducer involves relatively complicated construction, and an external device is required to open or close the cover provided in its rear chamber in order to achieve these exchangeable functions. Very few applications can be contemplated in which this exchangeable electroacoustic transducer can be used. Furthermore, its relatively complicated design makes production and use of the same difficult. The electroacoustic transducer disclosed in the '387 patent also does not provide adaptations which will allow it to be used by the hearing impaired.
U.S. Pat. No. 5,030,872 discloses an electroacoustic transducer for use as a transducer for telephones having a circular transducer plate arranged in a transducer housing, clamped between two mounting members at its edge region. At least one seating region of the mounting member is of a rotationally asymmetrical shape to attenuate partial oscillations of a higher order. The primary objective of the '872 patent is to attenuate the partial oscillations of the transducer plate characterized by two circular nodal lines; it still involves relatively complicated design and does not provide provisions for the hearing impaired.
Various improvements of piezoelectric receivers are also disclosed in German patents Nos. 3,107,293, 3,402,657, and in Japanese patents Nos. 3-54998, 3-54999, 60-265299, 61-89799, 61-89800, etc. All of the prior art piezoelectric receivers share the same common structure in that they use a pair of metallic outer housings to shield the transducer plates, which are supported by disc-shaped or ring-shaped projections, to form the main body of the receiver. The common drawbacks are also present in all of these above mentioned prior art piezoelectric receivers in that they involve relative complicated designs, thus prohibiting a meaningful reduction in the manufacturing cost. Furthermore, none of the prior art piezoelectric receivers are adaptable for use by the hearing impaired.